Engineers utilize knowledge of mechanical properties to determine the strength, durability, and safety of load bearing structures. Non-destructive testing (NDT) methods are appealing because they allow for the determination of these properties without damaging the structure as a whole during testing. They are used extensively in quality control of new manufacturing and diagnostics of existing structures.
Current industrial non-destructive techniques are limited to measuring the hardness of a material by indentation, which provides an index of a material's resistance to penetration by a hard indentor. Although indentation testing is widely used, it provides limited and uncertain information that often may not be used by engineers to confidently predict the existing strength or remaining life of structures.
Scratch testing has historically been used in a similar manner to indentation hardness measurement. The Moh's hardness scale is a measure of scratch hardness that has been used for over a century to evaluate resistance of materials to scratching. This test provides a qualitative measure of strength that only allows for comparisons between known materials. However, implementing this methodology with existing technology necessitates sophisticated equipment and time-consuming operations to perform the tests and collect the scratch response of the material.
Currently, scratch testing is used to measure the strength of thin-films and coatings. This is done by using a hard tip to scratch the material while controlling the load being applied until failure occurs. This testing method is limited to select applications where materials utilize thin-films or coatings. This restriction makes the technology unsuited for assessing mechanical properties of common engineering materials.
Other scratch testing devices and systems have been developed, but their underlying test apparatus are either too complex or not sufficiently accurate for broad commercial use. Although the scientific basis for predicting yield strength and ductility of metals through a scratch test has been proven, the existing testing systems provide only partial solutions for evaluating mechanical properties.
The use of scratch testing in industrial applications is limited due to several factors, including the complexity of controlling and measuring the numerous parameters associated with the test, as well as the lack of integrated and affordable equipment and resources to perform the test and analyze the data. The preexisting test techniques involve controlling the applied load during scratching and later measuring the depth of penetration and height of the material pile-up on each side of the scratch or the scratch width. Another preexisting technique to determine the scratch width is direct imaging of the scratch with a microscope or magnifying device. The preexisting methods include many processes and techniques, deterring the widespread use of scratch testing in industrial applications.
Further, with preexisting technology, surface referencing is accomplished by performing an initial scan with a tip at low contact load to map the surface profile. This operation may be carried out prior to, or after the scratch test is performed. In other words, the preexisting methods include two separate operations: scratch formation and surface referencing.